Method of improving the stress corrosion resistance of a susceptible aluminum alloy



United States Patent M 3,507,032 METHOD OF IMPROVING THE STRESS CORRO- SIONRESISTANCE OF A SUSCEPTIBLE ALUMI- NUM ALLOY George S. Foerster, Midland, Mich., assignor to The Dow Chemical Company, Midland, Mich., a corporation of Delaware No Drawing. Filed Aug. 25, 1967, Ser. No. 663,241 Int. Cl. 1522f 3/24 US. Cl. 29-4205 6 Claims ABSTRACT OF THE DISCLOSURE The present disclosure concerns a method of improving the stress corrosion resistance of a susceptible aluminum base alloy containing magnesium and zinc comprising fabricating the susceptible alloy in close homogeneous contact with a compatible insensitive aluminum base alloy, i.e., relatively insensitive to stress corrosion, by not working the mixture.

The. high strength and poor stress corrosion resistance of aluminum alloys containing primarily zinc and magnesium are well known. It is often impossible therefore to fully utilizetheir high strength because of their susceptibility to stress corrosion.

A primary object of the present invention, therefore, is to provide a novel method of improving the stress corrosion resistance of susceptible aluminum alloys.

A further object of the invention is to provide a method of improving the maximum allowable stress in aluminum alloys containing zinc and magnesium where corrosion is significant.

The above and other objects and advantages of the present invention are obtained in a method comprising a sequence of steps wherein, a stress corrosion susceptiible aluminum base alloy containing over 4% weight percentzinc, and over about 1.5 weight percent magnesium, the balance being essentially aluminum, is produced in a particulate form, e.g., by atomizing or flaking. The particulated susceptible alloy is then coated by one or more means with aluminum or an aluminum alloy which is not susceptible to stress corrosion and is compatible with the susceptible alloy. The term compatible as used herein with reference to a non-susceptible alloy or metal means thatsuch material will not significantly or detrimentally alter or, impair the workability, such as, for example, the extrudability, and hot rollability without cracking of the susceptible alloy to any appreciable extent. While the present method does tend to reduce the tensile properties somewhat, this is more than olfset in added design utility by the significant increase in stress corrosion resistance.

The coating may be accomplished, e.g. by vapor deposition, peen plating, using fine powder and binder, extrusion, hot rolling, and the like. The compatible alloy may also be added in particulate form and mixed with the particualte susceptible alloy. In this case, more of the compatible alloy may be needed. In any event, a means or method of coating the susceptible alloy or providing intimate contact of a compatible alloy with it will be used to providei as many points of contact between the compatible material and susceptible alloy as possible and preferably as continuous a coating as possible on the particulate susceptible alloy after fabrication. Only a very thin coating is needed. While not intending to be limiting, the, coating thickness of compatible alloy on the susceptible alloy may lie within a range' of from about 0.1 to about 10 thousands of an inch, it begin preferred to have or provide a coating as aforesaid as thin and as continu- 3,507,032 Patented Apr. 21, 1970 ous as possible. Up to about 50 percent by weight (of the alloy mixture) of the compatible alloy or metal may be employed. If aluminum metal is employed it should be used generally in lesser amounts than the compatible aluminum alloy as not to detrimentally affect the strength of the resultant alloy mixture. For example, up to about 15 percent of the aluminum may be employed with the susceptible alloy in the alloy mixture.

The coated mixture may be hot worked to form a dense strong shape by, e.g., extrusion, forging, rolling and/or compaction. In the case of extrusion, the emerging extrude may be quenched at theextruder and aged or subsequently solution heat treated, quenched, and aged. In either case, exposure at elevated temperature is generally minimized to avoid excessive dilution of the base alloy and loss of strength, or any detrimental enrichment of the coating in Zn and Mg which may cause stress corrosion. Some dilution of the base or susceptible alloy during hot working and heat treatment is normally to be expected. As is evident from the foregoing, certain of the hot working techniques which may be employed herein, e.g., extrusion, act as well as a means for coating the compatible alloy or metal onto the susceptible alloy while fabricating or hot working it.

Compatible metals or alloys include for example primary and pure aluminum, a more dilute stress corrosion resistant aluminum base alloy containing less than about 4% zinc and less than about 1.5 percent magnesium, aluminum-base alloys containing less than about 5% magnesium, aluminum-base alloys containing magnesium and silicon, any of the foregoing with one or more dispersion hardener elements therein, aluminum-base alloys containing manganese, chromium, iron, misch metal, titanium, or zirconium, and other compatible aluminumbase alloy systems.

Exemplary of compatible metals and alloys include, e.g. aluminum base plus 3.5 percent zinc plus 1.5 percent magnesium, the same alloy plus 2 percent manganese, aluminum base plus 3 percent manganese, aluminum base plus 3 percent manganese plus 3 percent magnesium, aluminum base plus 1 percent magnesium plus 0.75 silicon, aluminum base plus 1 percent magnesium plus 2 percent silicon plus 1 percent manganese, aluminum base plus 1 percent magnesium.

In general a compatible material is one which does not significantly or detrimentally alter or impair the workability of the susceptible alloy such as by, e.g., significantly decreasing melting point or adversely affecting its extrudability, hot rollability as to cracking and does not make it more difficult to work in general, while itself being substantially free of detrimental stress corrosion problems.

If desired, the compatible metal alloy may also be made anodic to the susceptible alloy to protect it electrochemically. This can be accomplished, for example, by adding noble metals, for instance, copper. Thus, because of its copper content, No. 7075 aluminum alloy, e.g., is cathodic to simple aluminum-zinc-magnesium alloys. The particulate mixtures are hot worked to form a dense, strong shape. This is readily accomplished, e.g., by extrusion, but other forms of hot working can also be employed.

In atomizing the susceptible alloy intended to be improved, the alloy may be atomized through a nozzle hydraulically to form fine droplets, or, through a nozzle with an atomizing fluid such as an inert gas to form fine droplets which solidify or freeze almost instantaneously. A gas jet atomizing technique may be employed also wherein a fine stream of molten metal is formed and then blown into fine droplets with cross jets of an inert gas such as argon, or natural gas. Other techniques of atomizing the susceptible alloy of the present invention,

such as disc atomizing disclosed in U.S. Patent 2,699,576, as well as any other molten alloy or metal herein, may also be employed to provide a pellet particle size of from about +20 mesh up to about 325 mesh (U.S. Standard Sieve Series) and preferably particles finer than about 100 mesh. This latter range may be considered and called herein the fine mesh fraction of unclassified atomized pellets.

In extruding the various particulate materials of the method of the present invention, the pellets are normally, but not necessarily, preheated in the extrusion container to a temperature within the general range of from about 600 to about 1000 F. The material is then subjected to pressure to die express it through an aperture to provide the effect and fabricated shape desired, with or without first pre-compacting the pellets. The extrusion may be water quenched at the press and aged, or, subsequently solution heat treated for from 1 minute to 4 hours at 700 to 1000 F., quenched, and aged for l to 72 hours at 200 to 400 F. In any case, exposure at elevated temperatures is generally minimized to avoid excessive dilution of the susceptible alloys being treated by the method of the present invention and loss of strength.

The following examples serve to provide illustrative and preferred embodiments of the present invention and are not to be construed as limiting the invention thereto.

To illustrate the low stress corrosion resistance of a susceptible alloy as described hereinbefore atomized pellets were provided of such alloy and then contacted in a coating manner by extruding the mixture with a compatible alloy in accordance with the present invention to increase the stress corrosion resistance of the susceptible alloy. The conditions and results including the appropriate data is presented hereinafter along with a detailed It can readily be seen from the information presented in the table that the present invention provides an outstanding increase in the stress corrosion resistance of the susceptible alloys in 1(a) and 2(a) by treatment with the compatible alloy and metal of 1(b) and 2(b) in accordance with the present invention. This vast improvement in stress corrosion resistance, moreover, was obtained with only a modest reduction in the tensile properties of the susceptible alloy. This small disadvantage is more than offset by the tremendous improvement as aforesaid in the stress corrosion resistance of the otherwise susceptible alloy.

The present invention may be modified and changed Without departing from the spirit or scope thereof and the invention'is only limited as defined in the appended claims.

I claim:

1. A method of improving the stress-corrosion resistance of a stress-corrosion susceptible aluminum base metal alloy containing over about 4 percent by weight of zinc and over about 1.5 weight percent of magnesium, the balance being essentially aluminum, comprising (a) particulating the stress-corrosion susceptible alloy and (b) hot working the susceptible alloy in substantially homogenous contact with up to about 50 weight percent of a metal selected from the group consisting of substantially pure aluminum or an aluminum base alloy which is relatively insensitive to stress corrosion, thereby improving the stress corrosion resistance of said susceptible alloy.

2. The method of claim 1 wherein the particulated susceptible alloy is coated by the insensitive metal to provide said homogeneous contact.

3. The method of claim 1 wherein, the particulated susceptible alloy and the insensitive metal are particulated 35 descrrptlon thereof. and mixed to provide said homogenous contact.

TABLE 1000 p.s.i., percent Life at 50,000 p.s.i. (hrs) Example Alloy Composition E TYS TS Parallel Pendieular 1 (a) .;rl-bl2\14se+10% Zn+2% Mg 7 90 92 28 3 n. (b) 75% of (a) +25% of Al- 84 87 78-166 6-22 base alloy containing 3.5% Zn+1.5% Mg+2% Mn. 2 (a) Al-base+6% Zn+2% Mg 6 93 100 30-46 54-70 +2% u 2 0 Mn. (b) 92% of (a) +8% fine Al- 82 91 1 44 198-214 powder (Reynolds 3OXD).

1 Days.

Item 1(-b) was prepared by first mixing atomized pellets, most of which ranged from to 200 mesh in size, comprising 75 percent by weight of the susceptible aluminum-base alloy of 1(a) and percent of compatible aluminum-base alloy 1(b) which contained 3.5 percent zinc, 1.5 percent magnesium and 2.0 percent manganese.

Item 2(b) was prepared by first mixing 92 percent of the atomized 20 to 200 mesh pellets of 2(a) with 8 percent of pure (99.3%) aluminum powder (pigment grade) of Reynolds Metals Co., XD (Reynolds Aluminum Data Bookl961).

All the alloys were extruded in /8 inch diameter rod at 800 F. and 5 feet per minute from a 3 inch container, solution heat treated one-half hour at 870 F., quenched in water, and aged 2 days at 250 F.

C-rings having 0.600 inch OD. and 0.481 I.D. were machined from the extrusion and stressed at 50,000 p.s.i. in the short transverse direction. The C-rings were tested with their axes either parallel or perpendicular to the extrusion axis and were loaded by tightening a screw which operated to close the C. All but the highly stressed center of the C was coated with plastic to eliminate electrochemical effects. The C-rings were then subjected to alternate-immersion in 3 /2 percent NaCl solution (10 minutes 1'n50 minutes out) at room temperature for a period in hours until fracture occurred.

' sensitive metal on the susceptible alloy is within a thickness range of from about 0.001 to 0.010 inch.

References Cited UNITED STATES PATENTS 1,580,647 4/1926 Breck 29420.5 2,398,132 4/ 1946 Cottrell 29-528 X 2,657,129 10/1953 Stern et al 117100 X JOHN F. CAMPBELL, Primary Examiner D. C. REILEY, Assistant Examiner U.S. Cl. X.R. 

